Printing apparatus and printing method using multiple nozzle groups

ABSTRACT

A print head  2  has multiple nozzle groups  2   a,    2   b  spaced apart in the sub-scanning direction by a prescribed inter-group distance pn i . Each nozzle group  2   a,    2   b  has multiple nozzles aligned in the sub-scanning direction at a nozzle pitch k. In a first printing scheme in which the nozzle groups record different raster lines, the inter-group distance pn i  is set to a value different from the nozzle pitch k. Since the nozzle pitch k need be secured only within the nozzle groups  2   a,    2   b  individually, a print head  2  with many nozzles can be readily obtained. The sub-scan feed amounts can be set to be a constant value or to be a combination of different values.

This application is a C-I-P of Ser. No. 09/337,401 filed Jun. 22, 1999,which is a continuation Ser. No. 09/039,252 filed Mar. 16, 1998, nowU.S. Pat. No. 5,946,011.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a printing apparatus and a printingmethod using multiple nozzle groups applicable to, for example, aninkjet serial- or line-printer, particularly to a printing apparatusequipped with a print head having multiple dot forming element groupsarrayed at different group spacing from the dot forming element pitchand to a printing method therefor.

2. Description of the Related Art

Conventional printing apparatuses include, for example, the serialprinter, which prints characters one by one, and the line printer, whichprints a line of characters at one time. The serial nonimpact inkjetprinter, for instance, provides a printout corresponding to the printdata by driving a print head formed with multiple nozzles in the mainscanning direction while jetting ink drops from the nozzles andconveying a printing paper or other printing medium in the sub-scanningdirection perpendicular to the main scanning direction. Since thisconventional inkjet printer forms adjacent dot lines on the printingmedium with ink drops jetted from the same nozzles, however, the effectof variance in nozzle characteristics and the like is conspicuous andthe print quality low.

This problem is addressed by, for example, U.S. Pat. No. 4,198,642,which teaches interlace printing with constant pitch sub-scanning, i.e.,a printing scheme in which the number of used nozzles n and he nozzlepitch k are set to be relatively prime and paper feed is effected at aconstant sub-scan amount of n dot pitch. Two integers are relativelyprime when they do not have common divisors other than 1.

FIG. 1 is a diagram for explaining conventional interlace printing. Theprint head 100 has N nozzles #1-#9 arrayed in the sub-scanning directionat a prescribed nozzle pitch k·D (N=9 and k=4 in the illustratedexample). Sub-scan feed of a printing paper is conducted at a constantfeed amount L·D. In the example shown in FIG. 1, since all of thenozzles are used to jet ink drops, the number of nozzles N and thenumber of used nozzles n is the same. D denotes the printing resolutionand is called the “dot pitch.” Regarding the various parameters definedas integer multiples of the dot pitch D (k·D, L·D etc.) in the followingdescription, only their integer portions are sometimes used. Forinstance, k may be called the “nozzle pitch” and L the “feed amount.”When interlace printing is conducted, the nozzle pitch k and thesub-scan feed amount L (=n) are relatively prime. For example, if k=4and the printing resolution in the sub-scanning direction is 360 dpi,the nozzle pitch k is 4 dots (4/360 inch). Similarly, the paper feedamount, i.e., the sub-scan feed amount L (=n) is 9 dots (9/360 inch).

As shown in FIG. 1, effecting a sub-scan of L dot pitch once every mainscan of the print head 100 causes adjacent dot lines to be printed bydifferent nozzles. For instance, the dot line formed by the first mainscan pass of nozzle #7 is followed by a dot line formed by nozzle #5,which is followed by a dot line formed by nozzle #3, which is followedby a dot line formed by nozzle #1. Interlace printing can thereforeproduce high-quality printed images since it spreads out the effect ofnozzle characteristic variance.

In the conventional inkjet printer of the interlace printing type, it istaken for granted that a constant nozzle pitch k can be secured. Basedon this assumption, the nozzle pitch k and the number of driven nozzlesn are set to be relatively prime and paper feed is conducted at aconstant pitch of n dots.

Demand for higher printing speeds in recent years has heightened theneed to form print heads with larger numbers of nozzles. Consistentformation of many nozzles at a constant nozzle pitch is, however,difficult. Nozzle pitch is apt to change midway and defects are likelyto occur in some of the nozzles. When the prescribed nozzle pitch cannotbe obtained, the quality of interlace printing by the prior art ismarkedly degraded owing to overwriting and/or skipping of raster lines.Since the prescribed nozzle pitch has to be secured in print heads,increase of the nozzle number will lower the production yield and raisethe production cost proportionally. The prior art thus does not takeinto account the recent need for the larger number of nozzles. Since theprior art printing technique requires a constant nozzle pitch for allthe nozzles of the nozzle head, it cannot be applied withoutmodification to a printing apparatus having a large number of nozzlesfor which a constant nozzle pitch is difficult to obtain.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to reproduce ahigh-quality printed image using a print head with a large number of dotforming elements. Another object of the present invention is to enableinterlace printing even when the pitch of the dot forming elementschanges midway.

In order to attain the above and other objects of the present invention,there is provided a printing apparatus that effects printing by formingdots in a print region on a printing medium. The printing apparatuscomprises: a print head; a first scan driver which moves at least one ofthe print head and the printing medium in a first scanning direction; asecond scan driver which moves at least one of the print head and theprinting medium in a second scanning direction perpendicular to thefirst scanning direction; and a print head driver which drives the printhead to form dots on the printing medium responsive to print image data.The print head includes N number (N being an integer not smaller than 4)of dot forming elements, where a minimum element pitch in the secondscanning direction between a neighboring pair of the dot formingelements is k·D (k being an integer; D being a dot pitch correspondingto printing resolution) in the print head. The N number of dot formingelements is classified into M number of dot forming element groups eachincluding N/M number of dot forming elements (M and N/M being integersnot smaller than 2), and an ith (i being an integer between 1 and (M−1))dot forming element group and an (i+1)th dot forming element group amongthe M number of dot forming element groups are offset in the secondscanning direction by an inter-group pitch pg_(i)·D (pg_(i) being aninteger different from k). The first and second scan drivers and theprint head driver drive the print head and the printing medium so thatthe M number of dot forming element groups have identical patterns ofdot-formable positions and the identical patterns of the M number of dotforming element groups are shifted from each other to make all dotpositions in the print region to be dot-formable.

The term “dot forming element” here denotes a mechanism which forms dotson the printing medium, including for example an inkjet type actuatorusing a piezoelectric vibrator or heater to jet ink drops fromapertures.

Since this printing apparatus can effect interlace printing at a feedamount of 2·D or greater using M number of dot forming element groups,it can produce high-quality printed images using a print head equippedwith a large number of dot forming elements.

The second scan driver may convey at least one of the print head and theprinting medium in the second scanning direction using a combination ofdifferent feed amounts. Alternatively, The second scan driver may conveyat least one of the print head and the printing medium in the secondscanning direction at a constant feed amount that is at least twice thedot pitch D.

According to an aspect of the present invention, each neighboring pairof the dot forming element groups are spaced apart by an interval in thesecond scanning direction, and the N/M number of dot forming elements ofeach dot forming element group are capable of forming NIM number ofidentical dots aligned substantially in a single row in the secondscanning direction at the minimum element pitch k·D.

In a preferred embodiment, the identical patterns of the M number of dotforming element groups are composed of multiple first scanning directiondot lines occurring periodically at a pitch of M dots.

The ith and (i+1)th dot forming element groups are separated by aninter-group distance pn_(i)·D (pn_(i) being an integer) and pn_(i) isset so that each of (M−1) number of remainders of dividing anaccumulated value (Σpn_(i)) of the values pn₁ to pn_(i) by M takes adifferent value between 1 and (M−1).

When, in this way, N number of dot forming elements are grouped into Mnumber of dot forming element groups and the inter-group distancepn_(i)·D of the dot forming element groups is set in the foregoingmanner, it suffices to establish a prescribed minimum element pitch k·Dwithin the individual dot forming element groups. In other words, aprint head having a large number of dot forming elements can be readilyobtained by integrating dot forming element groups having dot formingelements aligned at a prescribed minimum element pitch k·D.

The print head may be formed by arraying M number of dot forming elementunits separated in the second scanning direction by the inter-groupdistance pn_(i)·D, each dot forming element unit having the N/M numberof dot forming elements whose pitch in the second scanning direction isequal to the minimum element pitch k·D.

By using multiple dot forming element units having dot forming elementswith an element pitch k·D, a print head having a larger number of dotforming elements than heretofore can be readily obtained. Specifically,higher yield and lower production cost can be achieved when the printhead is formed by arraying multiple dot forming element units than whenit is formed by incorporating a large number of dot forming elementsinto the print head at one time.

Each dot forming element unit may have a row of even-numbered dotforming elements and a row of odd-numbered dot forming elements eachhaving multiple dot forming elements aligned in the second scanningdirection at an element pitch 2k·D which is twice the minimum elementpitch k·D, where the row of even-numbered dot forming elements and therow of odd-numbered dot forming elements are spaced from each other inthe first scanning direction.

By arranging two rows of dot forming elements side by side in the firstscanning direction, the element pitch of each row of dot formingelements can be made twice (=2k·D) that in the case of forming only asingle row. A large number of dot forming elements can therefore beeasily formed in a single dot forming element unit.

The first scan driver may drive the at least one of the print head andthe printing medium in the first scanning direction at a first scanningdirection speed that is a function of the number of scans S.

When the number of scans S is set at 2 (S=2), for instance, everycontinuous dot line in the first scanning direction is formed by 2scans. The printing speed is therefore halved if the print head feedspeed (the first scanning direction speed) is the same as when S=1. Theprint head feed speed is therefore dynamically varied as a function ofthe number of scans S to enable production of a high-quality printedimage without lowering the printing throughput. More specifically,“first scanning direction speed that is a function of the number ofscans S” means a first scanning direction speed proportional to thenumber of scans S. Although the first scanning direction speed ispreferably in proportion to the number of scans S is doubled, theinvention is not limited to this.

According to another aspect of the present invention, the identicalpatterns of the M number of dot forming element groups are composed ofmultiple dots occurring periodically at a pitch of M dots on every firstscanning direction dot line.

The ith and (i+1)th dot forming element groups may be separated by aninter-group distance pn_(i)·D where pn_(i) is an integer and at leastone of pn_(i) is different from k.

When N number of dot forming elements are grouped into M number of dotforming element groups and the inter-group distance pn_(i)·D of the dotforming element groups is set in the foregoing manner, it suffices toestablish a prescribed minimum element pitch k·D within the individualdot forming element groups. In other words, a print head having a largenumber of dot forming elements can be readily obtained by integratingdot forming element groups having dot forming elements aligned at aprescribed minimum element pitch k·D.

The M number of dot forming element groups may be formed by inactivatingat least one dot forming element in the print head among the multipledot forming elements arrayed in the second scanning direction at theminimum element pitch k·D.

In other words, multiple dot forming element groups may be obtained bynot using at least one of a plurality of dot forming elements all ofwhich are arrayed at a prescribed minimum element pitch k·D in thesecond scanning direction. In this case, the inter-group distancepn_(i)·D is a multiple of the minimum element pitch k·D. With thisarrangement, interlace printing according to the invention can beeffected when, for example, some dot forming elements are out ofservice, of degraded quality or otherwise defective by inactivating thedefective dot forming element(s).

According to still another aspect of the present invention, the N numberof dot forming elements are separated into BN number of blocks (BN beingequal to N/M) each including M number of dot forming elements, aneighboring pair of the BN number of blocks being separated by aninter-block distance pb·D (pb being a positive integer unequal to k),the M number of dot forming element groups being composed ofcorresponding dot forming elements in the blocks. The M number of dotforming elements of each block are capable of forming M number ofidentical dots aligned substantially in a single row in the secondscanning direction at the minimum element pitch k·D.

Consider, for example, the case where 10 dot forming elements aredivided into two blocks (N=10, BN=2). Since each block is formed with 5dot forming elements (N/BN=10/2=5), each block has first to fifth dotforming elements. By grouping the corresponding dot forming elements ofthe blocks, e.g., by grouping the first dot forming elements with eachother, the second dot forming elements with each other and the third dotforming elements with each other, it is possible to configure five dotforming element groups. When dot forming element groups are configuredin this manner, overlap printing can be effected by the interlacemethod.

The BN number of blocks may formed by inactivating at least one dotforming element in the print head among the multiple dot formingelements arrayed in the second scanning direction at the minimum elementpitch k·D

The first scan driver may drive the at least one of the print head andthe printing medium in the first scanning direction at a first scanningdirection speed that is a function of the number of scans M·S.

Here the M number of dot forming element groups each scans the same dotline S times. When two dot forming element groups M1 and M2 are used,for example, each dot line in the print region is scanned by the firstdot forming element group M1 and is also scanned by the second dotforming element group M2. The two scans by the dot forming elementgroups M1, M2 complete forming a continuous dot line in the firstscanning direction. Since S designates the number of scans of each dotforming element group it can be called the “number of group scans S.”

When S is set at 2 (S=2), for instance, each continuous dot line in thefirst scanning direction is formed by 2M scans. The printing speed istherefore halved if the print head feed speed (the first scanningdirection speed) is the same as when S=1. The print head feed speed istherefore adaptively varied as a function of the number of scans M·S toenable production of a high-quality printed image without lowering theprinting throughput.

More specifically, “first scanning direction speed that is a function ofthe number of scans M·S” means that the first scanning direction speedincreases with the number of scans M·S. Although the first scanningdirection speed is preferably in proportion to the number of scans M·S,the invention is not limited to this.

The present invention is also directed to a printing method that effectsprinting by forming dots in the print region on the printing medium. Themethod comprises the steps of providing a print head including N number(N being an integer not smaller than 4) of dot forming elements, aminimum element pitch in the second scanning direction between aneighboring pair of the dot forming elements being k·D (k being aninteger; D being a dot pitch corresponding to printing resolution) inthe print head, the N number of dot forming elements being classifiedinto M number of dot forming element groups each including N/M number ofdot forming elements (M and N/M being integers not smaller than 2), anith (i being an integer between 1 and (M−1)) dot forming element groupand an (i+1)th dot forming element group among the M number of dotforming element groups being offset in the second scanning direction byan inter-group pitch pg_(i)·D (pg_(i) being an integer different fromk); and driving the print head and the printing medium so that the Mnumber of dot forming element groups have identical patterns ofdot-formable positions and the identical patterns of the M number of dotforming element groups are shifted from each other to make all dotpositions in the print region to be dot-formable.

The present invention is also directed to a computer program productcomprising: a computer readable medium; and a computer program stored onthe computer readable medium. The computer program comprises a programfor causing the computer to produce print data for driving the printhead and the printing medium so that the M number of dot forming elementgroups have identical patterns of dot-formable positions and theidentical patterns of the M number of dot forming element groups areshifted from each other to make all dot positions in the print region tobe dot-formable.

These and other objects, features, aspects, and advantages of thepresent invention will become more apparent from the following detaileddescription of the preferred embodiments with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing interlace printing by the priorart.

FIGS. 2(A) and 2(B) are explanatory diagrams for demonstrating the basicconditions of a printing scheme using one nozzle group.

FIGS. 3(A) and 3(B) are explanatory diagrams for demonstrating the basicconditions of a printing scheme when the number of scans S is 2 orgreater.

FIG. 4 is an explanatory diagram for demonstrating the basic conditionsof a first printing scheme using multiple nozzle groups.

FIG. 5 is an explanatory diagram for demonstrating the basic conditionsof a second printing scheme using multiple nozzle groups.

FIG. 6 is a schematic view showing the overall configuration of aprinting apparatus according to a first embodiment of the first printingscheme of the invention.

FIG. 7 is a plan view showing the structure of a print head.

FIG. 8 is a sectional view showing the structure of the print head.

FIG. 9 is an explanatory view showing how print processing is conductedaccording to the first embodiment of the first printing scheme.

FIG. 10 is an enlarged explanatory view showing the condition of dotformation in FIG. 9.

FIG. 11 is an explanatory view showing how print processing is conductedby a printing apparatus according to a second embodiment of the firstprinting scheme.

FIG. 12 is an enlarged explanatory view showing the condition of dotformation in FIG. 11.

FIG. 13 is an explanatory view showing how print processing is conductedby a printing apparatus according to a third embodiment of the firstprinting scheme.

FIG. 14 is an enlarged explanatory view showing the condition of dotformation in FIG. 13.

FIG. 15 is an explanatory view showing how print processing is conductedby a printing apparatus according to a fourth embodiment of the firstprinting scheme of the invention.

FIG. 16 is an explanatory view showing how print processing is conductedby a printing apparatus according to a fifth embodiment of the firstprinting scheme.

FIG. 17 is an explanatory view showing how print processing is conductedaccording to a first embodiment of the second printing scheme of theinvention.

FIG. 18 is an enlarged explanatory view showing the condition of dotformation in FIG. 17.

FIG. 19 is an explanatory view showing how print processing is conductedby a printing apparatus according to a second embodiment of the secondprinting scheme.

FIG. 20 is an enlarged explanatory view showing the condition of dotformation in FIG. 19.

FIG. 21 is an explanatory view showing how print processing is conductedby a printing apparatus according to a third embodiment of the secondprinting scheme.

FIG. 22 is an enlarged explanatory view showing the condition of dotformation in FIG. 21.

FIG. 23 is an explanatory view showing how print processing is conductedby a printing apparatus according to a fourth embodiment of the secondprinting scheme of the invention.

FIG. 24 is an explanatory view showing the structure and other aspectsof the print head of a printing apparatus according to a fifthembodiment of the second printing scheme.

FIG. 25 is an enlarged explanatory view showing the condition of dotformation in FIG. 24.

FIG. 26 is a diagram for explaining the basic conditions of a firstirregular-feed printing scheme using multiple nozzle groups.

FIG. 27 is a diagram for explaining the basic conditions of a secondprinting scheme conducting irregular feed using multiple nozzle groups.

FIG. 28 is diagram for explaining the parameters of a first embodimentof the fist irregular-feed printing scheme.

FIG. 29 is an explanatory diagram showing how the print processing iscarried out in the first embodiment of the first irregular-feed printingscheme.

FIG. 30 is diagram for explaining the parameters of a second embodimentof the first irregular-feed printing scheme.

FIG. 31 is an explanatory diagram showing how the print processing iscarried out in the second embodiment of the first irregular-feedprinting scheme.

FIG. 32 is diagram for explaining the parameters of a third embodimentof the first irregular-feed printing scheme.

FIG. 33 is an explanatory diagram showing how the print processing iscarried out in the third embodiment of the first irregular-feed printingscheme.

FIG. 34 is diagram for explaining the parameters of a first embodimentof the second irregular-feed printing scheme.

FIG. 35 is an explanatory diagram showing how the print processing iscarried out in the first embodiment of the second irregular-feedprinting scheme.

FIG. 36 is diagram for explaining the parameters of a second embodimentof the second irregular-feed printing scheme.

FIG. 37 is an explanatory diagram showing how the print processing iscarried out in the second embodiment of the second irregular-feedprinting scheme.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A. Basic Conditions of Ordinary Printing Scheme

A-1. Basic Conditions of Printing Scheme Using one Nozzle Group.

FIGS. 2(A) and 2(B) are explanatory diagrams for demonstrating the basicconditions of a printing scheme using one nozzle group. The solid linecircles including numerals in FIG. 2(A) indicate the positions of 4nozzles in the sub-scanning direction after each sub-scan feed. Theencircled numerals 1-4 signify the nozzle numbers.

Various parameters related to the printing scheme are shown in FIG.2(B). The parameters of the printing scheme include the nozzle pitch k[dot], number of used nozzles N1, number of scans S, and sub-scan feedamount L [dot]. The number of scans S indicates the number of sub-scansin which all dots of a given raster line are serviced. In the example ofFIG. 2, since the dots of every raster line are serviced by onesub-scan, S=1.

In the example of FIGS. 2(A) and 2(B), the nozzle pitch k is 3 dots andthe number of used nozzles N1 is 4. (The number of used nozzles N1 isthe number of nozzles actually used among the multiple nozzlesprovided.) The number of scans S indicates that dots are formedintermittently once every S dots on a raster line during a single mainscan. The number of scans S is therefore equal to the number of nozzlesused to record all dots of each raster line.

The table in FIG. 2(B) shows, for each sub-scan feed, the sub-scan feedamount L, the accumulated value ΣL thereof, and the nozzle offset Fafter sub-scan feed. The offset F is a value indicating the distance innumber of dots between the nozzle position and reference positions ofoffset 0. The reference positions are presumed to be those periodicpositions having a pitch k, which include the initial positions of thenozzles where no sub-scan feed has been conducted (every third dot inFIG. 2(A)). For example, as shown in FIG. 2(A), the first sub-scan feedmoves the nozzles in the sub-scanning direction by the sub-scan feedamount L (4 dots). Since the nozzle pitch k is 3 dots, the offset F ofthe nozzles after the first sub-scan feed is 1 (see FIG. 2(A)).Similarly, the position of the nozzles after the second sub-scan feed isΣL (=8) dots away from the initial position so that the offset F is 2.The position of the nozzles after the third sub-scan feed is ΣL (=12)dots away from the initial position so that the offset F is 0. Since thethird sub-scan feed brings the nozzle offset F back to 0, all dots ofthe raster lines within the print region can be serviced by repeatingthe cycle of 3 sub-scans.

As will be understood from the foregoing example, when the nozzleposition is a position that is an integer multiple of the nozzle pitch kaway from the initial position, the offset F is 0. Further, the offset Fis given by (ΣL) % k, where ΣL is the accumulated value of the sub-scanfeed amount L , k is the nozzle pitch, and “%” is an operator indicatingthat the remainder of the division is taken. (Viewing the initialposition of the nozzles as being periodic, the offset F can also beviewed as the amount of phase shift of the nozzles from the initialposition.)

When the number of scans S is 1 and the sub-scan feed amount L isconstant, the following condition C1 must be satisfied to avoid skippingand/or overwriting of raster lines to be recorded.

[Condition C1]:

Sub-scan feed amount L equal to number of used nozzles N1, and sub-scanfeed amount L (=N1) and nozzle pitch k relatively prime.

Condition C1 can be understood as follows. When recording is effectedwith no skipping of raster lines, N1×k raster lines are recorded duringk sub-scans. After the k number of sub-scan feeds, the nozzle positionought to be N1×k raster lines away from the initial nozzle position.This nozzle position can be attained by making “the sub-scan feed amountL equal to the number of used nozzles N1.” To avoid skipping and/oroverwriting of the raster lines to be recorded, it is necessary for theoffsets F during the k number of sub-scan feeds to assume mutuallydifferent values between 0 and (k−1). Such values of the offsets F canbe ensured by making “the sub-scan feed amount L (=N1) and nozzle pitchk relatively prime.” Two integers are said to be “relatively prime” ifthey have no common divisors other than 1. Skipping and overwriting ofraster lines to be recorded can be avoided by satisfying condition C1.

FIGS. 3(A) and 3(B) are explanatory diagrams for demonstrating the basicconditions of a printing scheme when the number of scans S is 2 orgreater. The printing scheme shown in FIGS. 3(A) and 3(B) amounts tothat obtained by changing the number of scans S and the sub-scan feedamount L among the printing scheme parameters shown in FIG. 2(B). Aswill be understood from FIG. 3(A), the sub-scan feed amount L in theprinting scheme of FIGS. 3(A) and 3(B) is a constant value of 2 dots. InFIG. 3(A) the positions of the nozzles after odd-numbered sub-scan feedsare indicated by diamonds. As shown on the right-hand side of FIG. 3(A),the dot positions recorded after odd-numbered sub-scan feeds are shiftedby one dot in the main scanning direction from the dot positionsrecorded after even-numbered sub-scan feeds. This means that theplurality of dots in any given raster line are recorded intermittentlyby each of 2 different nozzles. For example, the uppermost raster linewithin the print region is intermittently recorded every other dot bynozzle #3 after the first sub-scan feed and is thereafter intermittentlyrecorded every other dot by nozzle #1 after the fourth sub-scan feed.Thus, when the number of scans S is 2 or greater, each raster line isrecorded by S number of different nozzles.

The bottom line in the table of FIG. 3(B) shows the value of the offsetF after each of multiple sub-scans. The values of offsets F after thefirst to sixth sub-scan feeds include each value between 0 and 2 twice.

Since one raster line is ordinarily recorded by S number of scans henthe number of scans S is 2 or greater, the effective number of nozzlescan be considered to be N1/S. The sub-scan feed amount L can thereforebe set equal to the effective number of nozzles N1/S. Specifically, whenthe number of scans S is an integral value of 2 or greater, condition C1set out above can be rewritten as the following condition C1′.

[Condition Cl′]:

Sub-scan feed amount L equal to effective number of nozzles N1/S, andsub-scan feed amount L (=N1/S) and nozzle pitch k relatively prime.

Since the sub-scan feed amount L and the nozzle pitch k are alsorelatively prime in condition C1′, the offsets F after the k number ofsub-scan feeds assume mutually different values between 0 and (k−1), asshown in FIG. 3(B). Moreover, the offsets F after k×S number of sub-scanfeeds assume each value between 0 and (k−1), S number of times. (Thenumber of scans S is selected so that N1/S is an integer not smallerthan 1.)

Condition C1′ also holds when the number of scans S is 1. Condition C1′therefore holds generally with respect to a printing scheme that effectssub-scan feed at a constant feed amount L using one nozzle group,irrespective of the value of the number of scans S. In a case where thenumber of scans S is 2 or greater, however, the condition of mutualoffset among the recording positions of the nozzles recording a givenraster line must be satisfied.

A-2. Basic Conditions of Printing Scheme Using Multiple Nozzle Groups

FIG. 4 is an explanatory diagram for demonstrating the basic conditionsof a first printing scheme using multiple nozzle groups. M number ofnozzle groups NG₁−NG_(M) (M=3 in FIG. 4) have identically arrayednozzles, and each group has N1 number of nozzles disposed at a constantnozzle pitch k. The total number of nozzles N of the M number of nozzlegroups NG₁−NG_(M) is therefore equal to N1·M. The distance between theith nozzle group NG_(i) and the (i+1)th nozzle group NG_(i+1) (calledthe “inter-group distance”) is pn_(i) dots. The distance betweencorresponding nozzles of the ith nozzle group NG₁ and the (i+1)th nozzlegroup NG_(i+1) (called the “inter-group pitch”) is pg_(i) dots.

The raster lines recorded by the individual nozzle groups aredistinguished on the right-hand side of FIG. 4. As this shows, in thefirst printing scheme each nozzle group records different raster linesfrom the others and the raster lines recorded by any given nozzle groupoccur periodically at a pitch of M dots. (How these raster lines arerecorded will be described later in detail.) Specifically, in the firstprinting scheme all of the raster line groups recorded by the individualnozzle groups exhibit the same periodic pattern of arrangement at apitch of M dots. All dots in the print region are recorded by offsettingthis identical pattern little by little every nozzle group.

In the printing scheme of FIG. 4, since each nozzle group uses multiplenozzles arrayed at nozzle pitch k to record raster lines spaced at apitch of M dots, the sub-scan feed amount L is set to M times that ofthe feed amount N1/S for a single nozzle group, that is, M·N1/S=N/S.Moreover, since this printing scheme is thought to be equivalent to ascheme in which each nozzle group uses nozzles with a nozzle pitch of(k/M) to record raster lines at a pitch of one dot, the effective numberof nozzles N1/S is set to be prime relative to k/M. Condition C1′ canthen be rewritten as follows.

[Condition C2 a]:

Sub-scan feed amount L equal to M times effective number of nozzles N1/S(=N/S), and effective number of nozzles N1/S (=N/(M·S)) and (k/M)relatively prime.

When condition C2 a is satisfied, each nozzle group can record rasterlines spaced at a pitch of M dots. (The nozzle pitch k and the number ofnozzle groups M are selected to make (k/M an integer not smaller than1.) On the other hand, the raster line groups recorded by the nozzlegroups can be mutually offset little by little as shown on theright-hand side of FIG. 4 by satisfying the following condition C2 b.

[Condition C2 b]:

Each of (M−1) values of (Σpn_(i))% M takes a different value between 1and (M−₁). where (Σpn_(i)) is the accumulated value of the inter-groupdistances pn₁−pn_(i) from the first to the ith nozzle group (i being aninteger between 1 and (M−1)), and the operator “%” indicates anoperation of taking the remainder of division. As far as the inter-groupdistance pn_(i) satisfies condition C2 b, the values of the (M−1) numberof inter-group distances pn₁ −pn_(i) can be equal to each other.

The following condition C2 c, using the inter-group pitch pg_(i) insteadof the inter-group distance pn_(i) in condition C2 b, also holds.

[Condition C2 c]:

Each of (M−1) values of (Σpg_(i))% M takes a different value between 1and (M−1).

Condition C2 c is more general than condition C2 b because theinter-group pitch pg_(i) can be made smaller than the distance k·(N1−1)between the opposite ends of one nozzle group. That is, condition C2 bis a special case satisfying the more general condition C2 c.

FIG. 5 is an explanatory diagram for demonstrating the basic conditionsof a second printing scheme using multiple nozzle groups. In thisprinting scheme, each nozzle group records 1/M of all dots of everyraster line. In other words, the dots recorded by each nozzle groupoccur at a pitch of M dots in every raster line. (How these dots arerecorded will be described later in detail.) In this printing scheme,since each nozzle group records at every raster line, the same conditionholds regarding the sub-scan feed as in the printing scheme shown inFIGS. 3(A) and 3(B) using only one nozzle group.

[Condition C3 a]:

Sub-scan feed amount L equal to effective number of nozzles N1/S(=N/(M·S)), and sub-scan feed amount L (=N/(M·S)) and nozzle pitch krelatively prime.

Regarding the inter-group distance pn_(i), it suffices to satisfy thefollowing condition C3 b, which is less strict than condition C2 b.

[Condition C3 b]:

Inter-group distance pn_(i) a different value from nozzle pitch k.

Similarly, regarding the inter-group pitch pg_(i), it suffices tosatisfy the following condition C3 c, which is less strict thancondition C2 c.

[Condition C3 c]:

Inter-group distance pg_(i) a different value from nozzle pitch k.

In the printing scheme shown in FIG. 5, each raster line is recorded byM number of nozzle groups and each nozzle group effects recording of oneraster line in S number of scans. Since every raster line is recorded inM·S number of scans, (M·S) is called the “number of raster line scans.”The number of scans S of one nozzle group is called the “number of groupscans.”

Although in the example of FIG. 5 each dot line in the column direction(vertical direction) is recorded by one nozzle group, it is insteadpossible, as in the example of FIGS. 17 and 18 explained later, torecord each dot line in the column direction with different nozzlegroups. The dots recorded by each nozzle group occur at a pitch of Mdots in every raster line in this case, too, but the positions of thedots recorded by the nozzle group are progressively offset in the rowdirection every raster line. Specifically, in the second printing schemethe dots recorded by each nozzle group exhibit the same pattern ofperiodic arrangement at a pitch of M dots in each raster line and alldots in the print region are recorded by offsetting this identicalpattern little by little every nozzle group.

The term “dot line” is used in this specification to indicate either aseries of dots in the row (horizontal) direction (i.e. a raster line) ora series of dots in the column (vertical) direction.

In the first printing scheme explained earlier, each nozzle grouprecords raster lines at a pitch of M dots and records all dots in theseraster lines. In the second printing scheme, each nozzle records atevery raster line but records dots at a pitch of M dots in each rasterline. The first and second printing schemes are common in the point that“the recording positions of multiple nozzle groups form identicalrecording position patterns and the recording position patterns of themultiple nozzle groups are offset from each other to service all dotpositions in the print region.” In the first printing scheme the“identical recording patterns” are patterns composed of “raster linesspaced at a pitch of M dots” and in the second printing scheme arepatterns composed of “dots spaced at a pitch of M dots at every rasterline.”

B. Embodiments of First Constant-feed Printing Scheme

B-1. First Embodiment of First Printing Scheme

FIGS. 6-8 show an inkjet printer 1 as a printing apparatus according toa first embodiment of the first constant-feed printing scheme of theinvention. FIG. 6 is an explanatory diagram showing the overallconfiguration of the inkjet printer 1. The inkjet printer 1 comprises aprint head 2, a main scan driver 3, a sub-scan driver 4, a drivercontroller 5, a data storage section 6, a print head driver 7 and a mainscan speed management table 8, each of which will be explained later.(In this embodiment, “first scanning direction” refers to the mainscanning direction (lateral direction in the drawings) and “secondscanning direction” to the sub-scanning direction (vertical direction).

The print head 2 has a first nozzle group 2 a and a second nozzle group2 b spaced apart in the sub-scanning direction by a prescribedinter-group distance pn·D. The nozzle groups are also referred to as“dot forming element groups.” The inter-group distance pn·D is an timesthe dot pitch D, which corresponds to the printing resolution. When thenumber of nozzle groups M is 2 as in the case of FIG. 6, a naturalnumber that is not a multiple of 2 (i.e., an odd number) is selected asthe inter-group distance an.

As shown in FIG. 7, each of the nozzle groups 2 a, 2 b is configured asan actuator unit 10, or “dot forming element unit”, and each is equippedwith N1 number of nozzles as “dot forming elements” (N1=5 in theillustrated example). In other words, N number of nozzles (N=N1+N1=10)are separated into two nozzle groups 2 a, 2 b. The number of nozzles Nis an integer not smaller than 4.

The nozzles of each nozzle group 2 a, 2 b are spaced in the sub-scanningdirection at a nozzle pitch of k·D, or the “element pitch.” The nozzlepitch k·D is k times the dot pitch D, where k is an integral multiple ofthe number of nozzle groups M.

The main scan driver 3, or the “first scan driver” , drives the printhead 2 in a main scanning direction (laterally in FIG. 6) relative to aprinting medium SP consisting of a sheet of printing paper or the like.The sub-scan driver 4, or the “second scan driver”, effects driving toconvey the printing medium SP in the sub-scanning directionperpendicular to the main scanning direction (vertically in FIG. 6).

The driver controller 5 controls the amount, timing etc. of the drivingby the main scan driver 3 and the sub-scan driver 4 so as to move theprint head 2 in the main scanning direction. The driver controller 5also makes the conveyance amount of the printing medium SP by thesub-scan driver 4 equal to N/S times the dot pitch D, i.e., N·D/S,thereby enabling a constant pitch medium conveyance operation mode andeffecting control to form dots in accordance with the interlace printingscheme.

In order to form adjacent dot lines with different nozzles, theparameters N, M, S and k have to satisfy the condition of “N/(M·S) andk/M being relatively prime.” Since the product M·S of the number ofnozzle groups M and the number of group scans S is a factor of thenumber of nozzles N and the nozzle pitch k is a multiple of the numberof nozzle groups M, N/(M·S) and k/M are both integers. In the exampleshown in FIG. 6, when the number of scans S is made 1,N/(M·S)=10/(2·1)=5 and k/M=4/2=2, so that N/(M·S) and k/M are relativelyprime. It is noted that these parameters satisfy the conditions C2 a, C2b and C2 c.

The data storage section 6 consists of a memory for storing print datasupplied from a computer 9. A data block area (not shown) is formed inthe memory. The print head driver 7 supplies electric current to theprint head 2 based on the print data stored in the data storage section6. In response, the first nozzle group 2 a and the second nozzle group 2b jet ink onto the printing medium SP from prescribed nozzles to producea printout based on the print data.

The print data supplied from the computer 9 include raster data andsub-scanning feed data. The raster data represent what kind of dot is tobe formed at each pixel position. In binary printers, the raster data isexpressed in one bit per pixel and represents on/off state of each pixelposition. In multilevel printers, on the other hand, the raster data isexpressed in at least two bits per pixel and represents the size of dotto be formed. The sub-scanning feed data represent a sub-scanning feedamount after each pass of main scan. The sub-scanning feed data istransferred to the driver controller 5. Such raster data are typicallyproduced by a printer driver stored in a main memory of the computer 9.The printer driver is a computer program which is typically provided tothe users of the printers in the form of a computer program product suchas CD-ROMs and floppy disks. The printer driver is installed into a harddick 9 a of the computer 9.

The main scan speed management table 8 is used for dynamicallycontrolling the main scan speed VS, or the “first scan direction speed”,in accordance with the number of scans S. Specifically, the main scanspeed VS, i.e., the speed of print head 2, is stored in the main scanspeed management table 8 in association with different print modes withdifferent numbers of scans S. The main scan speed VS1 when the number ofscans S is 1, i.e., when every dot line in the main scanning directionis formed by a single scan, is defined as a reference speed and the mainscan speed VS increases with the number of scans S. Specifically, themain scan speed VS2 when S=2 is set to double the reference speed VS1and the main scan speed VS3 when S=3 is set to treble the referencespeed VS1. The invention is not limited to this, however, and, forexample, it is possible to set the main scan speed VS2 when S=2 to 1.5times the reference scan speed VS1 or some other value.

A specific example of the print head 2 will now be explained based onFIGS. 7 and 8. FIG. 7 is a plan view of the print head 2. The print head2 is composed of a plurality (two in FIG. 7) of actuator units 10. Theactuator units 10 are separated by an inter-group distance pn·D. Eachactuator unit 10 is formed with multiple nozzle actuators.

FIG. 8 is a sectional view of a nozzle actuator. A channel forming plate11 is formed with an ink chamber 12, an ink feed port 13 and a pressurechamber 14. Ink is fed from an external ink tank (not shown) to the inkchamber 12, and then to the pressure chamber 14 through the ink feedport 13. A vibration plate 15 is provided on the rear side surface ofthe channel forming plate 11 and is formed with an island portion 16. Apiezoelectric vibrator 17 is provided in contact with one side of theisland portion 16. The piezoelectric vibrator 17 is formed so as, forexample, to contract when electrically charged and expand whendischarged.

A nozzle plate 20 has nozzle orifices 21 associated with respectivenozzle actuators. The nozzle orifices 21 at each actuator unit 10 arespaced at nozzle pitch kD. As schematically shown in FIG. 7, the printhead 2 is completed by providing the nozzle plate 20 on the actuatorunits 10. It should be noted, however, that the present invention is notlimited to the foregoing arrangement. For example, it is possible toadopt instead a configuration which jets ink drops by means of bubblesgenerated in the ink with microheaters.

Since the nozzle actuators have a complicated structure involving inkchannel portions such as the pressure chamber 14 and the piezoelectricvibrator 17, it is difficult to incorporate a large number of the nozzleactuators into a single actuator unit 10 with high uniformity. Since inthis embodiment the print head 2 is constituted by arraying multipleactuator units 10, however, a print head 2 equipped with multiple nozzleactuators can be readily obtained.

The operation of this embodiment will now be explained based on FIGS. 9and 10. As explained in the foregoing, in this embodiment the number ofnozzle groups M=2, the number of nozzles N=10, the nozzle pitch k=4, theinter-group distance pn=5, the number of scans S=1, and the sub-scanfeed amount L=N.

During every main scan pass, the nozzles of the nozzle groups 2 a, 2 bcan form dots by jetting ink drops. Since a constant pitch sub-scan of Ndot pitch is effected every main scan, dot lines cannot be denselyformed in the sub-scanning direction until the print head 2 and theprinting medium SP have come into a prescribed positional relationship.Specifically, the position of nozzle #B3 at the third main scan pass P3is the start of the print region.

FIG. 10 is an enlarged explanatory view showing how dots are formed in asection of 12 dot lines from the start of the print region. As shown inFIG. 10, since the number of scans S is 1 in this embodiment, every dotline in the main scanning direction is formed in a single main scan.Moreover, dot lines adjacent in the sub-scanning direction are formed bydifferent nozzles.

The embodiment configured in this manner exhibits the following effects.

First, since the print head 2 is formed by grouping the multiple nozzles(nozzle actuators) into the multiple nozzle groups 2 a, 2 b andseparating the nozzle groups 2 a, 2 b by the inter-group distance pn·D,whose value is different from that of the nozzle pitch k, a print head 2equipped with multiple nozzles can be readily obtained. Specifically,the print head 2 can be produced at high yield and low production costbecause the nozzle pitch k need be secured only within the nozzle groups2 a, 2 b individually.

Second, since the number of used nozzles N, the number of nozzle groupsM, the number of scans S and the nozzle pitch k are selected to makeN/(M·S) and k/M relatively prime and sub-scanning is effected at aconstant pitch that is N/S times the dot pitch D, interlace printing canbe achieved with the print head 2 even if the head 2 is locallydifferent in nozzle pitch. Adjacent dot lines can therefore be formedwith different nozzles, and the effect of nozzle characteristic variancecan be spread out to effect high-quality printing.

Third, since the print head 2 is formed by arraying in the sub-scanningdirection multiple actuator units 10 each having multiple nozzleactuators serially aligned in the sub-scanning direction at a nozzlepitch k, print heads with many nozzles can be produced with highuniformity. In addition, print heads 2 with various numbers of nozzlescan be obtained simply by changing the number of actuator units 10 used.

B-2. Second Embodiment of First Printing Scheme

A second embodiment of the first printing scheme according to theinvention will now be explained based on FIGS. 11 and 12. (In theembodiments that follow, constituent elements identical with those ofthe first embodiment of the first printing scheme described above areassigned the same reference symbols as those in the first embodiment andwill not be explained further.) The feature characterizing thisembodiment is that the nozzles are separated into 3 nozzle groups.

Specifically, the print head 31 of this embodiment is composed of afirst nozzle group 31 a, a second nozzle group 31 b and a third nozzlegroup 31 c each having 3 nozzles arrayed at nozzle pitch k. The firstnozzle group 31 a and the second nozzle group 31 b are separated by afirst inter-group distance pn₁·D and the second nozzle group 31 b andthe third nozzle group 31 c are separated by a second inter-groupdistance pn₂·D. The parameters in this embodiment are: number of usednozzles N=9, number of nozzle groups M=3, number of scans S=1, nozzlepitch k=6, first inter-group distance pn₁=8 and second inter-groupdistance pn₂=5. Since N/(M·S)=9/(3·1)=3 and k/M=6/3=2, it follows thatN/(M·S) and k/M are relatively prime.

When the inter-group distance pn_(i) differs between different nozzlegroups as in this embodiment, the distances can be determined based onEquation 1.

pn₁=(pn₂ +α·M)  (Equation 1)

where α is an integer.

In other words, the first inter-group distance pn₁ is the value obtainedby adding a multiple of M to the second inter-group distance pn₂. Inthis embodiment, the first inter-group distance pn₁ is determined to bepn₁=(pn₂+α·M)=(5+1·3)=8. In general, however, it suffices to satisfy theaforesaid condition C2 b: “Each of (M−1) values of (Σpn_(i))% M being adifferent value between 1 and (M−1).”

In this embodiment, as shown in FIG. 11, the position of nozzle #B2 atthe third main scan pass P3 is the start of the print region. Dot linescan be densely formed in the sub-scanning direction from here. FIG. 12is an enlarged explanatory view showing how dots are formed in a sectionof 15 dot lines from the start of the print region. As shown in FIG. 12,dot lines adjacent in the sub-scanning direction are formed by differentnozzles.

The embodiment configured in this manner can therefore also achieve thesame effects as the first embodiment of the first printing schemedescribed earlier.

B-3. Third Embodiment of First Printing Scheme

A third embodiment of the first printing scheme according to theinvention will now be explained based on FIGS. 13 and 14. The featurecharacterizing this embodiment is that every dot line in the mainscanning direction is formed by two scans in the main scanningdirection.

The print head 41 of this embodiment comprises a first nozzle group 41 aand a second nozzle group 41 b spaced apart in the sub-scanningdirection by an inter-group distance pn·D. The nozzle groups 41 a, 41 bare respectively formed with 6 nozzles aligned in the sub-scanningdirection at a nozzle pitch k·D. The parameters in this embodiment are:number of used nozzles N=12, number of nozzle groups M=2, number ofscans S=2, nozzle pitch k=4 and inter-group distance pn=5. SinceN/(M·S)=12/(2·2)=3 and k/M=4/2=2, it follows that N/(M·S) and k/M arerelatively prime.

As shown in FIG. 13, the position of nozzle #B5 at the fifth main scanpass P5 is the start of the print region in this embodiment. Each dotline is formed by two main scans. FIG. 14 is an enlarged explanatoryview showing how dots are formed in a section of 12 dot lines from thestart of the print region.

As shown in FIG. 14, dot lines adjacent in the sub-scanning directionare also formed by different nozzles in this embodiment. In addition,since the number of main scanning direction scans S is 2 in thisembodiment, each continuous dot line in the main scanning direction isformed by two main scans. This means that dots adjacent in the mainscanning direction are formed by different nozzles. This printingpattern is called “overlap.”

Since each raster line is scanned twice, printing is not limited to theoverlap pattern shown in FIG. 14 but can also be effected in other typesof overlap patterns. In this case, the first main scan forms acontinuous dot line and the next main scan overlays new dots on thealready formed dots, thus enabling a higher level of multigradationprinting.

B-4. Fourth Embodiment of First Printing Scheme

A fourth embodiment of the first printing scheme according to theinvention will now be explained based on FIG. 15. The featurecharacterizing this embodiment is that multiple actuator units areoffset from each other in the main scanning direction as well.

As shown in FIG. 15, the print head of this embodiment is composed ofmultiple actuator units 51. Each actuator unit 51 is formed withmultiple nozzles aligned in the sub-scanning direction at a prescribednozzle pitch k.

Adjacent ones of the actuator units 51 are separated from each other inthe sub-scanning direction so that the distance between the nearestnozzles thereof is a prescribed inter-group distance pn·D. The actuatorunits 51 are further offset from each other in the main scanningdirection by a prescribed distance WL.

The embodiment of this configuration also provides a number of nozzlegroups equal to the number of actuator units 51. It can thereforeproduce the same effects as the first embodiment described earlier.Moreover, since the actuator units 51 are offset from each other in themain scanning direction so that they may partly overlap in thesub-scanning direction, the length of the print head in the sub-scanningdirection can be shortened.

B-5. Fifth Embodiment of First Printing Scheme

A fifth embodiment of the first printing scheme according to theinvention will now be explained based on FIG. 16. The featurecharacterizing this embodiment is that the print head is formed byarranging in the sub-scanning direction actuator units each having a rowof even-numbered nozzles and a row of odd-numbered nozzles.

Specifically, the print head 61 according to this embodiment is equippedwith, for example, four nozzle groups 62 spaced in the main scanningdirection. The respective nozzle groups 62 are assigned to handledifferent colors, such as black, cyan, magenta and yellow, and eachnozzle group 62 jets ink drops of a single color.

Each nozzle group 62 is composed of multiple actuator units 63 alignedin the sub-scanning direction. Each actuator unit 63 has a row ofeven-numbered nozzles 63 a and a row of odd-numbered nozzles 63 b. Thetwo rows are spaced from each other in the main scanning direction, eachconsisting of multiple nozzles aligned in the sub-scanning direction ata nozzle pitch 2k·D. Actuator units 63 adjacent in the sub-scanningdirection are disposed so that the nearest nozzles thereof have aprescribed inter-group distance pn·D.

The embodiment configured in this manner can also achieve the sameeffects as the first embodiment of the first printing scheme describedearlier. Owing to the large nozzle pitch, moreover, this embodimentenables ready production of a multinozzle, high-density print head atlow cost.

As is clear from the example of FIG. 16, the N1 number of nozzlesincluded in each nozzle group need not necessarily be disposed along asingle straight line. Any arrangement suffices that is capable offorming N1 number of dots substantially aligned in the sub-scanningdirection at a constant pitch k.

One skilled in the art will be able to make suitable changes, additions,variations, deletions and the like in respect of the embodiments of thefirst printing scheme described in the foregoing without departing fromthe scope of the present invention. For example, while embodiments wereexplained in which dots are formed along the main scanning directiondefined as the first scanning direction, this is not limitative and itis possible instead to adopt a configuration in which printing iseffected along the sub-scanning direction defined as the second scanningdirection.

Although a serial printer was taken as an example in the embodiments ofthe first printing scheme, application to a line printer or to afacsimile apparatus, a photographic apparatus or the like is alsopossible. Application to a multifunction printing apparatusincorporating various features, e.g., facsimile capability, is alsofeasible.

As is clear from the foregoing explanation, the first printing schemeaccording to the invention differentiates the inter-group distance anbetween the dot forming element groups from the element pitch k betweenthe dot forming elements in the dot forming element groups, therebyenabling ready formation of a print head having a large number of dotforming elements. In addition, since it selects the parameters N, M, Sand to make N/(M·S) and k/M relatively prime and conveys the printingmedium at a constant pitch that is N/S times the dot pitch D, interlaceprinting can be achieved even though the presence of the interveninginter-group distance an causes the element pitch of the dot formingelements to differ locally within the head.

C. Embodiments of Second Constant-feed Printing Scheme

C-1. First Embodiment of Second Printing Scheme

The second printing scheme can use a hardware configurationsubstantially the same as that of the first printing scheme shown inFIGS. 6-8. FIG. 17 is an explanatory view showing how print processingis conducted according to a first embodiment of the second constant-feedprinting scheme.

A print head 71 has a first nozzle group 71 a and a second nozzle group71 b, or “dot forming element groups,” spaced apart in the sub-scanningdirection by a prescribed inter-group distance pn·D. The intergroupdistance pn·D is an times the dot pitch D. A positive integer other thank can be selected as the inter-group distance an.

Each of the nozzle groups 71 a, 71 b is equipped with N1 number ofnozzles as “dot forming elements” (N=5 in the illustrated example) Inother words, N number of nozzles (N=N1+N1=10) are separated into twonozzle groups 71 a, 71 b.

The number of nozzles N is an integer not smaller than 4. The number ofnozzles N and number of nozzle groups M (an integer not smaller than 2)are unequal.

The conveyance amount of the printing medium SP by the sub-scan driver 4is N/(M·S) times the dot pitch D, i.e., N·D/(M·S). This constant pitchconveyance operation mode enables the interlace printing scheme.

In order to form adjacent dots with different nozzles, the parameters N,M, S and k have to satisfy the condition of “N/(M·S) and k beingrelatively prime.” Since the number of raster line scans M·S, i.e., theproduct of the number of nozzle groups M and the number of group scansS, is a factor of the number of nozzles N and the nozzle pitch k is apositive integer, N/(M·S) and k are both integers. When the number ofgroup scans S is 1 in the example shown in FIG. 17, N/(M S)=10/(2·1)=5and k=4, so that N/(M·S) and k are relatively prime. The number of groupscans S here refers to the number of scans executed by each nozzlegroup, and the number of raster line scans M·S is the number of scansfor forming one dot line in the main scanning direction (i.e., oneraster line) by the scans of the individual nozzle groups. Theseparameters satisfy the aforesaid conditions C3 a, C3 b and C3 c.

The print head driver 7 (FIG. 6) supplies electric current to the printhead 71 based on the print image data stored in the data storage section6. In response, the first nozzle group 71 a and the second nozzle group71 b jet ink onto the printing medium SP from prescribed nozzles toproduce a printout based on the print data.

In the second printing scheme, the main scan speed management table 8(FIG. 2) is used for dynamically controlling the main scan speed VS, orthe “first scan direction speed”, in accordance with the number ofraster line scans M·S in the main scanning direction. Specifically, themain scan speed VS, i.e., the speed of print head 71, is stored in themain scan speed management table 8 in association with different printmodes with different numbers of scans M·S. The main scan speed VS1 whenthe number of group scans S is 1, i.e., when every dot line in the mainscanning direction is formed by a single scan of a single nozzle group,is defined as a reference speed and the main scan speed VS increaseswith the number of group scans S. Specifically, the main scan speed VS2when S=2 is set to double the reference speed VS1 and the main scanspeed VS3 when S=3 is set to treble the reference speed VS1. Theinvention is not limited to this, however, and, for example, it ispossible to set the main scan speed VS2 when S=2 to 1.5 times thereference speed VS1 or some other value. Although the main scan speedpreferably increases also in proportion to the number of nozzle groupsM, it may not be dependent on the number of nozzle groups M and can beset as a function of only the number of group scans S.

As explained in the foregoing, in the embodiment shown in FIG. 17 thenumber of nozzle groups M=2, the number of nozzles N=10, the nozzlepitch k=4, the inter-group distance pn=5, the number of group scans S=1,and the sub-scan feed amount N/(M·S)=10/2=5.

During every main scan pass, the nozzles of the nozzle groups 71 a, 71 bcan form dots by jetting ink drops. Since a constant pitch sub-scan ofN/(M·S) dot pitch is effected after every main scan, dot lines cannot bedensely formed in the sub-scanning direction until the print head 71 andprinting medium SP have come into a prescribed positional relationship.Specifically, the position of nozzle #A4 at the first main scan pass P1is the start of the print region. Moreover, since the nozzle groups 71a, 71 b respectively effect interlace printing, both nozzle groups 71 a,71 b contribute to form every raster line. That is, since overlapprinting is effected by the second printing scheme of the invention,every raster line in the print region is formed by both the nozzlegroups 71 a, 71 b.

FIG. 18 is an enlarged explanatory view showing how dots are formed in asection of 10 dot lines from the start of the print region. As shown inFIG. 18, since the number of group scans S is 1 in this embodiment,every dot line in the main scanning direction is formed by one main scanof the first nozzle group 71 a and one main scan of the second nozzlegroup 71 b.

Specifically, each dot line is formed of dots printed by the firstnozzle group 71 a (indicated by squares) and dots printed by the secondnozzle group 71 b (indicated by circles). Moreover, dot lines adjacentin the sub-scanning direction are formed by different nozzles.

The embodiment configured in this manner exhibits the following effects.

First, since the print head 71 is formed by grouping the multiplenozzles (nozzle actuators) into the multiple nozzle groups 71 a, 71 band separating the nozzle groups 71 a, 71 b by the inter-group distancean, a value differing from the nozzle pitch k, a print head 71 equippedwith multiple nozzles can be readily obtained. Specifically, the printhead 71 can be produced at high yield and low production cost becausethe nozzle pitch k need be secured only within the nozzle groups 71 a,71 b individually.

Second, since the number of used nozzles N, the number of nozzle groupsM, the number of group scans S and the nozzle pitch k are selected tomake N/(M·S) and k relatively prime and sub-scanning is effected at aconstant pitch that is N/(M·S) times the dot pitch D, interlace printingcan be achieved with the print head 71 which is locally different innozzle pitch. Since adjacent dot lines can therefore be formed withdifferent nozzles, the effect of nozzle characteristic variance can bespread out to effect high-quality printing.

Third, since in this embodiment both of the nozzle groups 71 a, 71 bscans every raster line in the print region, overlap printing can beeffected.

Fourth, since the print head 71 is formed by arraying in thesub-scanning direction multiple actuator units each having multiplenozzle actuators serially aligned in the sub-scanning direction at anozzle pitch k, print heads with many nozzles can be produced with highuniformity. In addition, print heads 71 with various numbers of nozzlescan be obtained simply by changing the number of actuator units 10 used.

In particular, since the inter-group distance an can be any positiveinteger other than the nozzle pitch k and is subject to no otherrestriction, a multinozzle print head 71 can be easily obtained byintegrating multiple actuator units.

Since this embodiment effects “overlap” by offsetting the positions ofthe dots formed by the nozzle groups 71 a, 71 b by one dot in the mainscanning direction every sub-scanning direction dot line, it forms dotsin a checkered pattern. The pattern of dot formation is not limited tothis, however, and it is instead possible, as in the second embodimentdescribed below, to align the dot forming positions of each nozzle groupin the sub-scanning direction.

C-2. Second Embodiment of Second Printing Scheme

A second embodiment of the second printing scheme according to theinvention will now be explained based on FIGS. 19 and 20. The featurecharacterizing this embodiment is that the nozzles are divided into 3nozzle groups.

Specifically, the print head 81 of this embodiment is composed of afirst nozzle group 81 a, a second nozzle group 81 b and a third nozzlegroup 81 c each having 3 nozzles arrayed at nozzle pitch k. Eachadjacent pair of nozzle groups, i.e., the first and second nozzle groups81 a and 81 b and the second and third nozzle groups 81 b and 81 c , isseparated by an inter-group distance pn·D. The parameters in thisembodiment are: number of used nozzles N=9, number of nozzle groups M=3,number of group scans S=1, nozzle pitch k=4 and inter-group distancepn=5. Since N/(M·S)=9/(3·1)=3 and k=4, it follows that N/(M·S) and k arerelatively prime.

Although the inter-group distance separating the first nozzle group 81from the second nozzle group 81 b and the inter-group distanceseparating the second nozzle group 81 b from the third nozzle group 81 care both defined as the an in this embodiment, the inter-group distancesare not required to be equal and either can be any integer other than k.This is because the nozzle groups 81 a, 81 b, 81 c each independentlyscans all the raster lines to effect interlace printing.

In this embodiment, as shown in FIG. 19, the position of nozzle #A1 atthe third main scan pass P3 is the start of the print region. Dot linescan be densely formed in the sub-scanning direction from here. FIG. 20is an enlarged explanatory view showing how dots are formed in a sectionof 8 dot lines from the start of the print region. As shown in FIG. 20,dot lines adjacent in the sub-scanning direction are formed by differentnozzles.

The embodiment configured in this manner can therefore also achieve thesame effects as the first embodiment of the second printing schemedescribed earlier.

C-3. Third Embodiment of Second Printing Scheme

A third embodiment of the second printing scheme according to theinvention will now be explained based on FIGS. 21 and 22. The featurecharacterizing this embodiment is that every dot line in the mainscanning direction is formed by two scans of each nozzle group in themain scanning direction.

The print head 91 of this embodiment comprises a first nozzle group 91 aand a second nozzle group 91 b spaced apart in the sub-scanningdirection by an inter-group distance pn·D. The nozzle groups 91 a, 91 bare respectively formed with 6 nozzles aligned in the sub-scanningdirection at a nozzle pitch k·D. The parameters in this embodiment are:number of used nozzles N=12, number of nozzle groups M=2, number ofgroup scans S=2, nozzle pitch k=4 and inter-group distance pn=5. SinceN/(M·S)=12/(2·2)=3 and k=4, it follows that N/(M·S) and k are relativelyprime.

As shown in FIG. 21, the position of nozzle #A5 at the third main scanpass P3 is the start of the print region in this embodiment. Each dotline is formed by two main scans of each of the nozzle groups 91 a, 91b. Since each of the nozzle groups 91 a, 91 b scans each raster linetwice (S=2), every line is composed of 4 kinds of dots, two kindsindicated by blank and solid circles and two kinds indicated by blankand solid squares.

FIG. 22 is an enlarged explanatory view showing how dots are formed in asection of 6 dot lines from the start of the print region. As shown inFIG. 22, dots adjacent in the sub-scanning direction are also formed bydifferent nozzles in this embodiment. In addition, since the number ofgroup scans S is defined as 2 in this embodiment, each continuous dotline in the main scanning direction is formed by two main scans by eachof the nozzle groups 91 a, 91 b. In every dot line, therefore, dotsadjacent in the main scanning direction are formed by a combination offour different nozzles.

C-4. Fourth Embodiment of Second Printing Scheme

A fourth embodiment of the second printing scheme according to theinvention will now be explained based on FIG. 23. The featurecharacterizing this embodiment is that a single actuator unit is usedand the nozzles are grouped into multiple nozzle groups by inactivatingsome nozzles.

Specifically, the print head 101 of this embodiment is formed of asingle actuator unit 102 having multiple nozzles aligned in thesub-scanning direction at a prescribed nozzle pitch k·D. In thisembodiment, a prescribed nozzle, the nozzle 103 indicated by a brokenline, is inactivated to divide the nozzles into a first nozzle group 101a and a second nozzle group 101 b.

The inter-group distance an between the nozzle groups 110 a, 101 bresulting from the inactivation the prescribed nozzle 103 is twice thenozzle pitch k.

The embodiment configured in this manner also achieves the same effectsas the first embodiment of the second printing scheme described earlier.In addition, since this embodiment is configured to effect the interlaceprinting that is a feature of this invention by inactivating at leastone nozzle to divide the total number of nozzles into the multiplenozzle groups 101 a, 101 b, interlace printing can be effected even whenthe actuator unit 102 has a missing or defective nozzle by selecting themissing or defective nozzle as an inactivated nozzle.

C-5. Fifth Embodiment of Second Printing Scheme

A fifth embodiment of the second printing scheme according to theinvention will now be explained based on FIGS. 24 and 25. The featurecharacterizing this embodiment is that N number of nozzles are separatedinto BN number of blocks and that M number (M=N/BN) of nozzle groups areconfigured by nozzles at the corresponding places in the blocks.

FIG. 24 is an explanatory view showing the structure and other aspectsof the print head 111 of this embodiment. The print head 111 is formedwith N number (N=10) of nozzles separated into BN number (BN=2) ofblocks. The nozzle pitch k in each block is 4 and the inter-blockdistance pb between the blocks is 5. The physical layout of the nozzlesis therefore the same as that of the first embodiment of the secondprinting scheme shown in FIG. 17.

In this embodiment, however, the structural unit in terms of drivecontrol for controlling the driving of the nozzles, i.e., the nozzlegrouping, differs from that of the first embodiment. Since each blockincludes N/BN number (N/BN=10/2=5) of nozzles, the nozzles of each blockare located at 1st through (N/BN)-th places within the block.

This will be explained with reference to FIG. 24. The print head 111 iscomposed of two blocks 112 and 113, and each of the blocks 112, 113 hasfive nozzles. The nozzles in each block are labeled with a through e,for convenience of illustration. In other words, the first block 112 hasfive nozzles labeled a1 through e1 and the second block 113 has fivenozzles labeled a2 through e2.

In this embodiment, two nozzles of the corresponding places in theblocks 112, 113 form a nozzle group. Specifically, five nozzle groupsare formed: a first nozzle group 111 a consisting of the nozzles a1 anda2, a second nozzle group 111 b consisting of the nozzles b1 and b2, athird nozzle group 111 c consisting of the nozzles c1 and c2, a fourthnozzle group 111 d consisting of the nozzles d1 and d2, and a fifthnozzle group 111 e consisting of the nozzles e1 and e2.

In this embodiment, since the nozzle groups 111 a-111 e are configuredby nozzles of the corresponding places in the blocks 112, 113, thedistance between the two nozzles of each nozzle group (i.e., effectivenozzle pitch) is {k·(M−1)+pb}. Interlace printing can therefore beconducted by selecting N, M, S, k and pb to make N/(M·S) and{k·(M−1)+pb} relatively prime and effecting sub-scanning at a constantpitch of N/(M·S). In the example of FIG. 24, N=10, M=5 (N/BN=10/2=5),k=4, pb=5 and S=1, so that N/(M·S)=10/5=2 and{k·(M−1)+pb}={4·(5−1)+5}=21. N/(M·S) and {k·(M−1)+pb} are thereforerelatively prime. The sub-scan feed amount is 10/(5·1)=2 dots.

In the printing scheme shown in FIG. 24, the inter-group pitch is k andthe distance (pitch) between the two nozzles of each nozzle group is{k·(M−1)+pb}. Although this embodiment does not satisfy the aforesaidcondition C3 b regarding the inter-group distance, they satisfyconditions C3 a and C3 c.

As shown in FIG. 25, in the embodiment of the foregoing configuration,dot lines can be densely formed from where the second nozzle a2 of thefirst nozzle group 111 a is positioned at the eleventh main scan pass.

Although the nozzles of the blocks 112, 113 are respectively representedby circles and squares in FIG. 24, the blocks 112, 113 are notdifferentiated in printing control. FIG. 25 does not differentiate thenozzles by which block they belong to. The symbols within the circlesrepresenting the dots in FIG. 25 indicate the nozzle group concerned,the number of main scan passes and the nozzle number. For example,“b11-2” means that the dot was formed by the second nozzle (“b”) of thesecond nozzle group (“-2”) in the “11”th main scan pass.

Like the earlier embodiments, this embodiment can also form dot linesadjacent in the sub-scanning direction with different nozzles to therebyeffect high-quality printing.

The second printing scheme can also utilize the print head 51 (FIG. 15)of the fourth embodiment of the first printing scheme or the print head61 (FIG. 16) of the fifth embodiment of the first printing schemedescribed earlier.

One skilled in the art will be able to make suitable changes, additions,variations, deletions and the like in respect of the embodiments of thesecond printing scheme described in the foregoing without departing fromthe scope of the present invention. For example, while embodiments wereexplained in which dots are formed along the main scanning directiondefined as the first scanning direction, this is not limitative and itis possible instead to adopt a configuration in which printing iseffected along the sub-scanning direction defined as the second scanningdirection.

In the second printing scheme of the invention, since every dot formingelement group scans every raster line, printing is not limited to the“overlap” pattern indicated regarding the embodiments but can also beeffected in other types of overlap patterns. In this case, the firstmain scan forms a continuous dot line and the next main scan overlaysnew dots on the already formed dots, thus enabling a higher level ofmultigradation printing.

Although a serial printer was taken as an example in the embodiments ofthe second printing scheme, application to a line printer etc. or to afacsimile apparatus, a photographic apparatus or the like is alsopossible. Application to a multifunction printing apparatusincorporating various features, e.g., facsimile capability, is alsofeasible.

As is clear from the foregoing explanation, the second printing schemeaccording to the invention differentiates the inter-group distance anbetween the dot forming element groups from the element pitch k betweenthe dot forming elements in the dot forming element groups, therebyenabling ready formation of a print head having a large number of dotforming elements. In addition, since it selects the parameters N, M, Sand k to make N/(M·S) and k relatively prime and conveys the printingmedium at a constant pitch that is N/(M·S) times the dot pitch D,interlace printing can be achieved even though the presence ofintervening intergroup distance an causes the element pitch of the dotforming elements to differ locally within the head.

In the second printing scheme of the present invention, the intergroupdistance an can be any positive integer other than the nozzle pitch k.Since interlace printing can therefore be conducted even if theinter-group distance an differs between different dot forming elementgroups, a print head with many dot forming elements can be easilyobtained.

D. Basic Conditions of Irregular-feed Printing Scheme:

FIG. 26 is a diagram for explaining the basic conditions of a firstirregular-feed printing scheme using multiple nozzle groups. By“irregular feed” is meant a sub-scanning scheme that uses a combinationof feed amounts of different values. In contrast, the feed in theprinting scheme illustrated in FIGS. 4 and 5 is called “constant feed”because the feed amount is fixed.

The arrangement of the M number of nozzle groups NG₁−NG_(M) shown inFIG. 26 is the same as that of the constant-feed first printing schemeshown in FIG. 4. The conditions indicated at the bottom of FIG. 26differ from the conditions of the constant-feed first printing schemeshown in FIG. 4 only in the sub-scan feed amount Lj.

In the irregular-feed printing scheme, however, the number of scans S isnot limited to an integral value but can be selected as a valueincluding a decimal fraction. When the number of scans S is an integralvalue, overlap printing (printing in which formation of all dots iscompleted in two or more main scans) is conducted on all raster lines.When the number of scans S includes a decimal fraction, it may happenthat overlap printing is conducted on some raster lines and notconducted on the others. This type of printing is called “partialoverlap printing.” The irregular-feed printing scheme, being subject toless severe parameter restrictions than constant-feed printing, can alsoeffect partial overlap printing.

In the first irregular-feed printing scheme shown in FIG. 26, as in thefirst constant-feed printing scheme, each nozzle group records differentraster lines and the raster lines recorded by each nozzle group occurperiodically at a pitch of M dots. Specifically, in the firstirregular-feed printing scheme, the raster lines recorded by theindividual nozzle groups all exhibit the same pattern of periodicarrangement at a pitch of M dots and all dots in the print region arerecorded by offsetting this identical pattern little by little everynozzle group.

As was explained with reference to FIG. 3, however, in the constant-feedprinting scheme using a single nozzle group, condition C1′ set out againbelow is satisfied.

[Condition C′]:

Sub-scan feed amount L equal to effective number of nozzles N1/S, andsub-scan feed amount L (=N1/S) and nozzle pitch k relatively prime.

By satisfying condition C1′, the offsets F (F=(ΣL)% k) during the k·Snumber of sub-scan feeds will take every value between 0 and (k−1), Stimes each. All dots in the print region can therefore be serviced withnone being needlessly overwritten or skipped.

Although, condition C1′ cannot be satisfied in the irregular-feedprinting scheme, it can be seen from an analysis of condition C1′ andits effect that in the irregular-feed printing scheme it suffices to seta sub-scan feed amount Lj as follows.

[Condition C4]:

Average value ave(Lj) of sub-scan feed amount Lj during k·S scans equalto effective number of nozzles N1/S, and offset (ΣLj)% k at the jthsub-scan feed during k·S scans (j is an integer between 1 and k·S) takeseach value between 0 and (k−1), S times.

(ΣLj) indicates the cumulative sum of the first to jth feed amounts L₁to L_(j) and “%” is an operator indicating that the remainder of thedivision is taken.

Condition C4 can be applied when only a single nozzle group is used. Aswas explained with reference to FIG. 4, when M number of nozzle groupsare used, each nozzle group is responsible for recording raster linesoccurring at a pitch of M dots. It therefore suffices for the value of1/M times the sub-scan feed amount Lj to satisfy condition C4. Further,the value relating to the nozzle pitch k of condition C4 can beconsidered to have become substantially 1/M. Therefore, in the firstirregular-feed printing scheme, condition C4 can be rewritten as thefollowing condition C4 a.

[Condition C4 a]:

Average value ave(Lj/M) of sub-scan feed amount Lj per group duringk·S/M scans equal to effective number of nozzles N1/S, and value{(ΣLj/M)% (k/M)} at jth sub-scan feed during k·S/M scans (j is aninteger between 1 and k·S/M) takes each value between 0 and {k/M)−1)}, Stimes.

(ΣLj) indicates the cumulative sum of the first to jth feed amounts L₁to L_(j) and “%” is an operator indicating that the remainder of thedivision is taken.

By satisfying condition C4 a, each nozzle group can be made to scan eachof the raster lines occurring at a pitch of M dots S times. And all dotsin the print region can be serviced with none being needlesslyoverwritten or skipped.

The other conditions C2 b and C2 c of the first constant-feed printingscheme can be adopted without modification as irregular feed conditionsC4 b and C4 c.

[Condition C4 b]:

Each of (M−1) values of (Σpn_(i))% M takes a different value between 1and (M−1).

[Condition C4 c]:

Each of (M−1) values of (Σpg_(i) )% M takes a different value between 1and M−1).

Condition C4 c relating to the inter-group pitch pg_(i) is more moderatethan condition C4 b relating to the inter-group distance pn_(i). Thismeans that condition C4 b need not be satisfied so long as condition C4c is satisfied.

FIG. 27 is a diagram for explaining the basic conditions of a secondirregular-feed printing scheme. In the second irregular-feed printingscheme, as in the second constant-feed printing scheme shown in FIG. 5,every nozzle group records every raster line and each nozzle group isresponsible for recording 1/M of all dots of each raster line. In otherwords, the dots serviced by each nozzle group occur at a pitch of M dotsin every raster line. Since in this printing scheme every nozzle groupconducts recording on every raster line, the following condition C5 aholds with respect to the sub-scan feed amount Lj. Condition C5 a issimilar to condition C4 in the printing scheme using a single nozzlegroup explained earlier.

[Condition C5 a]:

Average value ave(Lj) of sub-scan feed amount Lj during k·S scans equalto effective number of nozzles N1/S, and offset (ΣLj)% k at jth sub-scanfeed during k·S scans (j is an integer between 1 and k·S) takes eachvalue between 0 and (k−1), S times.

The other conditions C3 b and C3 c of the second constant-feed printingscheme can be adopted without modification as irregular feed conditionsC5 b and C5 c.

[Condition C5 b]:

At least one inter-group distance pni takes a value different fromnozzle pitch k.

[Condition C5 c]:

At least one inter-group pitch pgi takes a value different from nozzlepitch k.

Condition C5 c relating to the inter-group pitch pg_(i) is more moderatethan condition C5 b relating to the inter-group distance pn_(i). Thismeans that condition C5 b need not be satisfied so long as condition C5c is satisfied.

Although in the example of FIG. 27 each dot line in the column direction(vertical direction) is recorded by one nozzle group, it is insteadpossible to record each dot line in the column direction with multiplenozzle groups. The dots recorded by each nozzle group occur at a pitchof M dots in every raster line in this case, too, but the positions ofthe dots recorded by the nozzle group are progressively offset in therow direction every raster line. Specifically, in the secondirregular-feed printing scheme the dots recorded by each nozzle groupexhibit the same pattern of periodic arrangement at a pitch of M dots ineach raster line and all dots in the print region are recorded byoffsetting this identical pattern little by little every nozzle group.

In the first and second irregular-feed printing schemes explained above,the sub-scan feed amount Lj can be set to various values. The conditionsregarding the number of nozzles used can therefore be relaxed comparedwith the case of constant feed. As this enables more of the installednozzles to be used for printing, the printing speed can be increased.The irregular-feed printing scheme also enables the combination ofnozzles used to record a given raster line to be changed by adjustingthe feed amount arrangement. This capability can be used to mitigate thebanding (occurrence of image-degrading streaks in the main scanningdirection) that arises when a bad combination of nozzles is used torecord a given raster line. The irregular-feed printing scheme is thuscapable of printing high-quality images at high speed.

Even using the same print heads, multiple types of sub-scan feed amountarrangements can be adopted to implement irregular feed. Therefore, inthe irregular-feed printing scheme, the parameters of the printingscheme can be readily set to obtain high image quality.

E. Embodiments of the Irregular-feed Printing Scheme

E-1. Embodiments of the First Irregular-feed Printing Scheme

FIG. 28 is diagram for explaining the parameters of a first embodimentof the first irregular-feed printing scheme and FIG. 29 is anexplanatory diagram showing how the print processing is carried out. Inthis printing scheme, the number of nozzle groups M=2, the number ofnozzles per group N1=6, the total number of nozzles N=12, the nozzlepitch k=4, the number of scans S=1, and the inter-group distance pn=5.Each sub-scan feed cycle includes 2 (=k·S/M) feeds. The table in FIG. 28further indicates the per-cycle sub-scan feed amount Lj, Lj/M(hereinafter called “per-group feed amount”), the cumulative sum Σ(Li/M)thereof, and the offset F (=(ΣLi/M)% (k/M)).

As shown at the bottom of FIG. 28, the average value ave (Li/M) of theper-group sub-scan feed amount Lj during each cycle is 6 and is equal tothe effective number of nozzles N1/S. The offset F takes each valuebetween 0 and 1 one time during each cycle. The printing scheme of FIG.28 therefore satisfies condition C4 a set out above and the inter-groupdistance an satisfies condition C4 b.

As shown in FIG. 29, each of the two nozzle groups 201 a and 201 bservices one of every two raster lines in the print region. As a result,all raster lines in the print region can be recorded with none skippedor needlessly overwritten.

FIG. 30 is diagram for explaining the parameters of a second embodimentof the first irregular-feed printing scheme and FIG. 31 is anexplanatory diagram showing how the print processing is carried out. Inthis printing scheme, the number of nozzle groups M=3, the number ofnozzles per group N1=4, the total number of nozzles N=12, the nozzlepitch k=6, the number of scans S=1, and the inter-group distancepn1=pn2=7. Each sub-scan feed cycle includes 2 (=k·(S/M feeds.

As shown at the bottom of FIG. 30, the average value ave(Lj/M of theper-group feed amount Lj/M during each cycle is 4 and is equal to theeffective number of nozzles N1/S. As shown in the table in FIG. 30, theoffset F takes each value between 0 and 1 one time during each cycle.The printing scheme of FIG. 30 therefore satisfies condition C4 a setout above and he inter-group distance an satisfies condition C4 b.

As shown in FIG. 31, each of the three nozzle groups 202 a-202 cservices one of every three raster lines in the print region. As aresult, all raster lines in the print region can be recorded with noneskipped or needlessly overwritten.

FIG. 32 is diagram for explaining the parameters of a third embodimentof the first irregular-feed printing scheme and FIG. 33 is anexplanatory diagram showing how the print processing is carried out. Inthis printing scheme, the number of nozzle groups M=3, the number ofnozzles per group N1=4, the total number of nozzles N=12, the nozzlepitch k=3, the number of scans S=2, and the inter-group distancepn1=pn2=7. Each sub-scan feed cycle includes 2 (=k·S/M feeds.

As shown at the bottom of FIG. 32, the average value ave (Lj/M) of theper-group feed amount Lj/M during each cycle is 2 and is equal to theeffective number of nozzles N1/S. As shown in the table in FIG. 32, theoffset F takes the value 0 two times during each cycle. The printingscheme of FIG. 32 therefore satisfies condition C4 a set out above andthe inter-group distance an satisfies condition C4 b.

As shown in FIG. 33, each of the three nozzle groups 203 a-203 cservices one of every three raster lines in the print region. Eachraster line is recorded using two different nozzles.

A blank square indicates a dot serviced during the first scan of theraster line by the first nozzle group 203 a and a solid square indicatesa dot serviced during the second scan of the same raster line by thefirst nozzle group 203 a . A solid triangle indicates a dot servicedduring the second scan of the raster line by the second nozzle group 203b and a solid circle indicates a dot serviced during the second scan ofthe raster line by the third nozzle group 203 c. The dot positionsserviced during the first scan and the dot positions serviced during thesecond scan can be interchanged.

E-2. Embodiments of the Second Irregular-feed Printing Scheme

FIG. 34 is a diagram for explaining the parameters of a first embodimentof the second irregular-feed printing scheme and FIG. 35 is anexplanatory diagram showing how the print processing is carried out. Inthis printing scheme, the number of nozzle groups M=2, the number ofnozzles per group N1=6, the total number of nozzles N=12, the nozzlepitch k=4, the number of scans S=1, and the inter-group distance pn=6.Each sub-scan feed cycle includes 4 (=k·S) feeds. The table in FIG. 34indicates the per-cycle sub-scan feed amount Lj, the cumulative sumΣLi/M thereof, and the offset F (=(ΣLi)% k).

As shown at the bottom of FIG. 34, the average value ave(Li) of the feedamount during each cycle is 6 and is equal to the effective number ofnozzles N1/S. The offset F takes each value between 0 and 3 one timeduring each cycle. The printing scheme of FIG. 34 therefore satisfiescondition C5 a set out above and the inter-group distance an satisfiescondition C5 b.

As shown in FIG. 35, each of the two nozzle groups 211 a and 211 bservices all raster lines in the print region. The first and secondnozzle groups 211 a and 211 b service alternate dots of each rasterline.

FIG. 36 is diagram for explaining the parameters of a second embodimentof the second irregular-feed printing scheme and FIG. 37 is anexplanatory diagram showing how the print processing is carried out. Inthis printing scheme, the number of nozzle groups M=2, the number ofnozzles per group N1=8, the total number of nozzles N=16, the nozzlepitch k=4, the number of scans S=2, and the inter-group distance pn=6.Each sub-scan feed cycle includes 8 (=k·S) feeds.

As shown at the bottom of FIG. 36, the average value ave(Li) of the feedamount during each cycle is 4 and is equal to the effective number ofnozzles N1/S. As shown in the table in FIG. 36, the offset F takes eachvalue between 0 and 3 two times during each cycle. The printing schemeof FIG. 36 therefore satisfies condition C5 a set out above and theinter-group distance an satisfies condition C5 b.

As shown in FIG. 37, each of the two nozzle groups 212 a and 212 bservices all raster lines in the print region. The first and secondnozzle groups 212 a and 212 b service alternate dots of each rasterline. A solid square indicates a dot serviced during the second scan ofthe raster line by the first nozzle group 212 a. A solid circleindicates a dot serviced during the second scan of the raster line bythe second nozzle group 212 b. Servicing of the dots on a raster line isthus completed in four scans.

In the second irregular-feed printing scheme, the main scan speedmanagement Table 8 (FIG. 6) dynamically controls the main scan speed VS,or the “first scan direction speed”, in accordance with the number ofraster line scans M·S. Specifically, the main scan speed VS, i.e., thespeed of print head 71, is stored in the main scan speed managementTable 8 in association with different print modes with different numbersof scans M·S. The main scan speed VS1 when the number of group scans Sis 1, i.e., when every dot line in the main scanning direction is formedby a single nozzle group in a single scan, is defined as a referencespeed and the main scan speed VS increases with the multiplying factorof the number of group scans S. Specifically, the main scan speed VS2when S=2 is set to double the reference speed VS1 and the main scanspeed VS3 when S=3 is set to treble the reference speed VS1. Theinvention is not limited to this, however, and, for example, it ispossible to set the main scan speed VS2 when S=2 to 1.5 times thereference scan speed VS1. Although the main scan speed is preferablyincreased in proportion as the number of nozzle groups M increases, itis not dependent on the number of nozzle groups M and can be varied inproportion to only the number of group scans S.

As the first and second irregular-feed printing schemes described in theforegoing employ multiple nozzle groups, a print head equipped with manynozzles can be readily utilized. Moreover, restrictions on the sub-scanfeed amount and the number of nozzles used is less severe than in thecase of constant feed because the sub-scan feed amounts need not be setto a fixed value but can be set to a combination of different values.Many nozzles can therefore be used to achieve higher printing speeds. Inaddition, the combination of nozzles used to record a given raster linecan be changed. This capability can be used to decrease banding(occurrence of image-degrading streaks in the main scanning direction)and thus to enhance image quality.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A printing apparatus that effects printing byforming dots in a print region on a printing medium, comprising: a printhead; a first scan driver which moves at least one of the print head andthe printing medium in a first scanning direction; a second scan driverwhich moves at least one of the print head and the printing medium in asecond scanning direction perpendicular to the first scanning direction;and a print head driver which drives the print head to form dots on theprinting medium responsive to print image data; wherein the print headincludes N number (N being an integer not smaller than 4) of dot formingelements, a minimum element pitch in the second scanning directionbetween a neighboring pair of the dot forming elements being k·D (kbeing an integer; D being a dot pitch corresponding to printingresolution) in the print head, the N number of dot forming elementsbeing classified into M number of dot forming element groups eachincluding N/M number of dot forming elements (M and N/M being integersnot smaller than 2), an ith (i being an integer between 1 and (M−1)) dotforming element group and an (i+1)th dot forming element group among theM number of dot forming element groups being offset in the secondscanning direction by an inter-group pitch pg_(i)·D (pg_(i) being aninteger different from k); and the first and second scan drivers and theprint head driver driving the print head and the printing medium so thatthe M number of dot forming element groups have identical patterns ofdot-formable positions and the identical patterns of the M number of dotforming element groups are shifted from each other to make all dotpositions in the print region to be dot-formable.
 2. A printingapparatus according to claim 1, wherein: the second scan driver conveysat least one of the print head and the printing medium in the secondscanning direction using a combination of different feed amounts.
 3. Aprinting apparatus according to claim 2, wherein each neighboring pairof the dot forming element groups are spaced apart by an interval in thesecond scanning direction, and the N/M number of dot forming elements ofeach dot forming element group are capable of forming N/M number ofidentical dots aligned substantially in a single row in the secondscanning direction at the minimum element pitch k·D.
 4. A printingapparatus according to claim 3, wherein: the identical patterns of the Mnumber of dot forming element groups are composed of multiple firstscanning direction dot lines occurring periodically at a pitch of Mdots.
 5. A printing apparatus according to claim 4, wherein: each firstscanning direction dot line is formed by S number of scans in the firstscanning direction where S is a positive integer; and the feed amountLj·D in the second scanning direction where j is an integer between 1and k·S/M is set such that an average value of Lj/M during (k·S)/M feedsis equal to N/(M·S) and that a remainder of dividing (ΣLj/M) by k/Mtakes each value between 0 and {(k/M)−1}, S times, where (ΣLj/M) denotesa cumulative sum of L1/M through Lj/M.
 6. A printing apparatus accordingto claim 5, wherein: the ith and (i+1)th dot forming element groups areseparated by an inter-group distance pn_(i)·D (pn_(i) being an integer)and An_(i) is set so that each of (M−1) number of remainders of dividingan accumulated value (Σpn_(i)) of the values pn₁ to pn_(i) by M takes adifferent value between 1 and (M−1).
 7. A printing apparatus accordingto claim 6, wherein: the print head is formed by arraying M number ofdot forming element units separated in the second scanning direction bythe inter-group distance pn_(i)·D, each dot forming element unit havingthe N/M number of dot forming elements whose pitch in the secondscanning direction is equal to the minimum element pitch k·D.
 8. Aprinting apparatus according to claim 7, wherein: each dot formingelement unit has a row of even-numbered dot forming elements and a rowof odd-numbered dot forming elements each having multiple dot formingelements aligned in the second scanning direction at an element pitch2k·D which is twice the minimum element pitch k·D, the row ofeven-numbered dot forming elements and the row of odd-numbered dotforming elements being spaced from each other in the first scanningdirection.
 9. A printing apparatus according to claim 6, wherein: thefirst scan driver drives the at least one of the print head and theprinting medium in the first scanning direction at a first scanningdirection speed that is a function of the number of scans S.
 10. Aprinting apparatus according to claim 3, wherein: the identical patternsof the M number of dot forming element groups are composed of multipledots occurring periodically at a pitch of M dots on every first scanningdirection dot line.
 11. A printing apparatus according to claim 10,wherein: each first scanning direction dot line is formed by S number ofscans in the first scanning direction where S is a positive integer; andthe feed amount Lj·D in the second scanning direction where j is aninteger between 1 and k·S is set such that an average value of Lj duringk·S feeds is equal to N/(M·S) and that a remainder of dividing (ΣLj) byk takes each value between 0 and (k−1), S tines, where (ΣLj) denotes acumulative sum of L1 through Lj.
 12. A printing apparatus according toclaim 11, wherein: the ith and (i+1)th dot forming element groups areseparated by an inter-group distance pn_(i)·D where pn_(i) is aninteger, at least one of pn_(i) being different from k.
 13. A printingapparatus according to claim 12, wherein: the print head is formed byarraying M number of dot forming element units separated in the secondscanning direction by the inter-group distance pn_(i)·D, each dotforming element unit having the N/M number of dot forming elements whosepitch in the second scanning direction is equal to the minimum elementpitch k·D.
 14. A printing apparatus according to claim 13, wherein: eachdot forming element unit has a row of even-numbered dot forming elementsand a row of odd-numbered dot forming elements each having multiple dotforming elements aligned in the second scanning direction at an elementpitch 2k·D which is twice the minimum element pitch k·D, the row ofeven-numbered dot forming elements and the row of odd-numbered dotforming elements being spaced from each other in the first scanningdirection.
 15. A printing apparatus according to claim 12, wherein: theM number of dot forming element groups are formed by inactivating atleast one dot forming element in the print head among the multiple dotforming elements arrayed in the second scanning direction at the minimumelement pitch k·D.
 16. A printing apparatus according to claim 12,wherein: the first scan driver drives the at least one of the print headand the printing medium in the first scanning direction at a firstscanning direction speed that is a function of the number of scans M·S.17. A printing apparatus according to claim 2, wherein: the N number ofdot forming elements are separated into BN number of blocks (BN beingequal to N/M) each including M number of dot forming elements, aneighboring pair of the BN number of blocks being separated by aninter-block distance pb·D (pb being a positive integer unequal to k),the M number of dot forming element groups being composed ofcorresponding dot forming elements in the blocks; and the M number ofdot forming elements of each block are capable of forming M number ofidentical dots aligned substantially in a single row in the secondscanning direction at the minimum element pitch k·D.
 18. A printingapparatus according to claim 17, wherein: the print head is formed byarraying BN number of dot forming element units separated in the secondscanning direction by the inter-block distance pb·D, each dot formingelement unit having the M number of dot forming elements whose pitch inthe second scanning direction is equal to the minimum element pitch k·D.19. A printing apparatus according to claim 18, wherein: each dotforming element unit has a row of even-numbered dot forming elements anda row of odd-numbered dot forming elements each having multiple dotforming elements aligned in the second scanning direction at an elementpitch 2k·D which is twice the minimum element pitch k·D, the row ofeven-numbered dot forming elements and the row of odd-numbered dotforming elements being spaced from each other in the first scanningdirection.
 20. A printing apparatus according to claim 17, wherein: theBN number of blocks are formed by inactivating at least one dot formingelement in the print head among the multiple dot forming elementsarrayed in the second scanning direction at the minimum element pitchk·D.
 21. A printing apparatus according to claim 17, wherein: the firstscan driver drives the at least one of the print head and the printingmedium in the first scanning direction at a first scanning directionspeed that is a function of the number of scans M·S.
 22. A printingapparatus according to claim 1, wherein the second scan driver conveysat least one of the print head and the printing medium at a constantfeed amount that is at least twice the dot pitch D.
 23. In a printingapparatus for forming dots on a printing medium responsive to printimage data while moving at least one of a print head and the printingmedium in a first scanning direction and moving at least one of a printhead and the printing medium in a second scanning directionperpendicular to the first scanning direction, a printing method thateffects printing by forming dots in the print region on the printingmedium, comprising the steps of: providing a print head including Nnumber (N being an integer not smaller than 4) of dot forming elements,a minimum element pitch in the second scanning direction between aneighboring pair of the dot forming elements being k·D (k being aninteger; D being a dot pitch corresponding to printing resolution) inthe print head, the N number of dot forming elements being classifiedinto M number of dot forming element groups each including N/M number ofdot forming elements (M and N/M being integers not smaller than 2), anith (i being an integer between 1 and (M−1)) dot forming element groupand an (i+1)th dot forming element group among the M number of dotforming element groups being offset in the second scanning direction byan inter-group pitch pg_(i)·D (pg_(i) being an integer different fromk); conveying at least one of the print head and the printing medium inthe second scanning direction; and driving the print head and theprinting medium so that the M number of dot forming element groups haveidentical patterns of dot-formable positions and the identical patternsof the M number of dot forming element groups are shifted from eachother to make all dot positions in the print region to be dot-formable.24. A printing method according to claim 23, wherein: the conveyance ofat least one of the print head and the printing medium in the secondscanning direction is performed using a combination of different feedamounts.
 25. A printing method according to claim 24, wherein eachneighboring pair of the dot forming element groups are spaced apart by adot-forming-element-free interval in the second scanning direction, andthe N/M number of dot forming elements of each dot forming element groupare capable of forming N/M number of identical dots alignedsubstantially in a single row in the second scanning direction at theminimum element pitch k·D.
 26. A printing method according to claim 25,wherein: the identical patterns of the M number of dot forming elementgroups are composed of multiple first scanning direction dot linesoccurring periodically at a pitch of M dots.
 27. A printing methodaccording to claim 26, wherein: each first scanning direction dot lineis formed by S number of scans in the first scanning direction where Sis a positive integer; and the feed amount Lj·D in the second scanningdirection where j is an integer between 1 and k·S/M is set such that anaverage value of Lj/M during (k·S)/M feeds is equal to N/(M·S) and thata remainder of dividing (ΣLj/M) by k/M takes each value between 0 and{(k/M)−1}, S times, where (ΣLj/M) denotes a cumulative sum of L1/Mthrough Lj/M.
 28. A printing method according to claim 27, wherein: theith and (i+1)th dot forming element groups are separated by aninter-group distance pn_(i)·D (pn_(i) being an integer) and pn_(i) isset so that each of (M−1) number of remainders of dividing anaccumulated value (Σpn_(i)) of the values pn₁ to pn_(i) by M takes adifferent value between 1 and (M−1).
 29. A printing method according toclaim 28, wherein: the at least one of the print head and the printingmedium is conveyed in the first scanning direction at a first scanningdirection speed that is a function of the number of scans S.
 30. Aprinting method according to claim 25, wherein: the identical patternsof the M number of dot forming element groups are composed of multipledots occurring periodically at a pitch of M dots on every first scanningdirection dot line.
 31. A printing method according to claim 30,wherein: each first scanning direction dot line is formed by S number ofscans in the first scanning direction where S is a positive integer; andthe feed amount Lj·D in the second scanning direction where j is aninteger between 1 and k·S is set such that an average value of Lj duringk·S feeds is equal to N/(M·S) and that a remainder of dividing (ΣLj) byk takes each value between 0 and (k−1), S times, where (ΣLj) denotes acumulative sum of L1 through Lj.
 32. A printing method according toclaim 31, wherein: the ith and (i+1)th dot forming element groups areseparated by an inter-group distance pn_(i)·D where pn_(i) is aninteger, at least one of pn_(i) being different from k.
 33. A printingmethod according to claim 32, wherein: the at least one of the printhead and the printing medium is driven in the first scanning directionat a first scanning direction speed that is a function of the number ofscans M·S.
 34. A printing method according to claim 24, wherein: the Nnumber of dot forming elements are separated into BN number of blocks(BN being equal to N/M) each including M number of dot forming elements,a neighboring pair of the BN number of blocks being separated by aninter-block distance pb·D (pb being a positive integer unequal to k),the M number of dot forming element groups being composed ofcorresponding dot forming elements in the blocks; and the M number ofdot forming elements of each block are capable of forming M number ofidentical dots aligned substantially in a single row in the secondscanning direction at the minimum element pitch k·D.
 35. A printingmethod according to claim 34, wherein: the at least one of the printhead and the printing medium is driven in the first scanning directionat a first scanning direction speed that is a function of the number ofscans M·S.
 36. A printing method according to claim 23, wherein theconveyance of at least one of the print head and the printing medium inthe second scanning direction is performed at a constant feed amountthat is at least twice the dot pitch D.
 37. A computer program productfor causing a computer to produce print data to be supplied to aprinting apparatus for forming dots on a printing medium responsive tothe print data while moving at least one of a print head and theprinting medium in a first scanning direction and moving at least one ofa print head and the printing medium in a second scanning directionperpendicular to the first scanning direction, the print head includingN number (N being an integer not smaller than 4) of dot formingelements, a minimum element pitch in the second scanning directionbetween a neighboring pair of the dot forming elements being k·D (kbeing an integer; D being a dot pitch corresponding to printingresolution) in the print head, the N number of dot forming elementsbeing classified into M number of dot forming element groups eachincluding N/M number of dot forming elements (M and N/M being integersnot smaller than 2), an ith (i being an integer between 1 and (M−1)) dotforming element group and an (i+1)th dot forming element group among theM number of dot forming element groups being offset in the secondscanning direction by an inter-group pitch pg_(i)·D (pg_(i) being aninteger different from k), the computer program product comprising: acomputer readable medium; and a computer program stored on the computerreadable medium, comprising: a program for causing the computer toproduce the print data for driving the print head and the printingmedium so that the M number of dot forming element groups have identicalpatterns of dot-formable positions and the identical patterns of the Mnumber of dot forming element groups are shifted from each other to makeall dot positions in the print region to be dot-formable.
 38. A computerprogram product according to claim 37, wherein: the print data includesecond-scanning feed data representing conveyance of at least one of theprint head and the printing medium in the second scanning directionusing a combination of different feed amounts.
 39. A computer programproduct according to claim 37, wherein: the print data includessecond-scanning feed data representing conveyance of at least one of theprint head and the printing medium in the second scanning direction at aconstant feed amount that is at least twice the dot pitch D.